Flexible feltable fibers of titanium nitride and their preparation



3,012,856 FLEXIBLE FELTABLE FIBERS F TITANIUM NITE AND THEE PREPARATIONKenneth L. Berry, Hockessin, Deh, assignor to E. I. du Pont de Nemoursand Company, Wilmington, Del., a corporationof Delaware No Drawing.Filed July 10, 1957, Ser. No. 670,879

7 Claims. (Cl. 23-191) This invention relates to new inorganic fibrousmaterials and to their preparation. More particularly this inventionrelates to a novel fibrous form of titanium nitride and to a process forits preparation.

Titanium ntiride has been known for a long time and has been the subjectof much investigation. It is generally bronze yellow in color and isresistant to attack by inorganic acids. It is particularly resistant tohigh temperature, having a melting point reported to be over 2900 C. Thenitride has a high electrical conductivity.

Titanium nitride has been obtained by the reaction of a titanium halideand nitrogen or of titanium oxide with a reducing agent and nitrogen.The processes heretofore employed have given titanium nitride in afinelydivided state or as a layer. Bars of sintered titanium nitride arereported to be brittle (Schwarzkopf and Kieffer, Refractory Hard-Metals,Macmillan, N.Y., 1953, page 233). This suggests a low-tensile strengthfor the material. No fibrous form of titanium nitride has been reportedand particularly none which is highly flexible.

It is an object of this invention to provide new inorganic fibrousmaterials and a process for their preparation. A further object is toprovide a novel fibrous form of titanium nitride and a process for itspreparation. Another object is to provide a fibrous form of titaniumnitride which is highly flexible. Still another object is to providetitanium nitride in the form of asbestos-like fibers having high tensilestrength and outstanding flexibility. Other objects will appearhereinafter.

These and other objects are accomplished'by the following inventionwhich comprises an inorganic asbestoslike flexible fiber consistingessentially of titanium nitride and characterized in that said fiber hasa diameter of less than about 10 microns and a flexibility sufficient topermit bending around a mandrel of 1 mm. diameter without breaking.There have now been prepared as new inorganic fibrous materials, theasbestos-like form of titanium nitride characterized in that it is inthe form of flexible fibers each having a diameter of less than about 10microns with a length of at least 10 times the diameter and aflexibility .sufficient to permit wrapping around a mandrel of 1 mm.diameter without breaking.

The fibrous product consisting predominantly of titanium nitride isobtained by passing a mixture of titanium tetrachloride and nitrogen ina slowly moving stream of not more than 300 linear cm./hour measuredunder standard conditions of temperature and pressure into contact witha siliceous material heated to a temperature of at least 1225 C. and inthe presence of a reducing agent, and removing the fibers that areformed.

The following examples illustrate the preparation and properties of thenew fibers.

Example I Pelleted channel black carbon (5.4 parts by weight) having aspecific surface area of 625 sq. meters/g. was placed in amullite-bonded (Al Si O alumina ceramic boat which, in turn, was placedin a mullite ceramic tube. Nitrogen flowing at a rate of approximately0.05 mole/hr. was dried to a dewpoint of about 80 C. by passage througha solid CO cooled vessel and then bubbled through titanium tetrachloridewhich vaporized 3,012,856 Patented Dec. 12, 1961 ice at a rate of0.00044 mole/hr. This gas mixture was slowly passed through the tubeduring heating for 4.5 hours at l4251450 C. After cooling of the productunder nitrogen flow, it was found that the surface of the carbon wascovered with burnished gold-colored fibers having a wide range ofdiameters from less than 1 micron to several microns and up to about 1cm. in length. The fibers were flexible and could be bent sharplywithout breakage. The X-ray diffraction pattern of the fibersindicatedthey were of titanium nitride- During this reaction, the boat(originally 12.8 parts) lost 1.5 parts and the contents showed a netloss of 0.8 part. Other products either in the boat or depositeddownstream there'- from were colorless fibers which gave an X-raydiffraction pattern corresponding to beta-silicon carbide and silica.

Repetition of the above general procedure with 2.182 g. of carbon, 1.7g. titanium tetrachloride for 5 hours at 1440-1450 C. gave fibroustitanium nitride and fibrous rutile.

When the above general procedure was repeated except that a temperatureof 1300 C. was used, fine fibers were likewise obtained.

Example II An alumina boat containing 0.85 g. of carbon black(Neospectra) was heated in a mullite (aluminum silicate) tube (1%"internal diameter x 30" long) to 1435 to 1450 C. in a stream ofnitrogen. The nitrogen stream flowing at a rate of 0.079 mole/hr. wasbubbled through heated titanium tetrachloride which vaporized at arateof 0.003 mole/hr. The titanium tetrachloride vapors in nitrogen werethen conducted through the mullite tube over the heated carbon black.After 6 hours, the titanium tetrachloride flow was stopped and the boatand its contents were allowed to cool to room temperatureunder nitrogen.The contents of the boat at the end weighed 0.94 g. and gave an X-raypattern corresponding to that of titanium nitride. The boat decreased inweight from 14.96 g. at the start of the experiment to 13.75 g. at theend. On the sides and surfaces of the boat was a growth ofbronze-colored fibers (0.04 g.) that gave an X-ray diffraction patternfor titanium nitride. These fibers were found to contain 15.94-14.78% N(two determinations). The fiber diameter was 0.05 to 1.0 micron and thelength was up to 5 mm.

" Example III A graphite boat containing 0.99 g. of powdered aluminacovered with a layer of carbon black weighing 0.49 g; was heated in amullite tube (1% internal diameter x 30" long) in a stream of nitrogento 1395-1450 C. Titanium tetrachloride (0.00069 mole/hr.) and nitrogenat 0.050 mole/hr; were passed over the hot charge for 6-8 hours. At theend of the experiment the tube was found to contain a downstream fiberplug (0.15 g.) that gave an X-ray diffraction pattern corresponding tobeta-SiC (cubic) and face centered cubic crystals of cell constant a4.26 A. (TiO and TiN have a 4.235 A.; TiC has n 4.32 A.). A deposit ofbrown to gray fibers above the boat at the upstream end weighed 0.09 g.and contained individual fibers as long as 1.0 cm. with a diameter of006-30 microns. These fibers gave an X-ray diffraction pattern indexedas face-centered cubic, a 4.26 A., indicative of a TiN fiber containingdissolved TiC and TiC. f

Example I V An alumina boat containing 5.64 g. of 20-30 mesh h 3mole/hr.) were passed over the hot charge for 7 hours. After allowingthe tube and its contents to cool to room temperature under nitrogen, itwas found that golden fibers had deposited on the tube walls above theboat (0.14 g.). The X-ray diffraction patterns of these fiberscorresponded to TiN probably containing some Al SiO The fiber diameterwas 0.58 to 2.0 microns and the length ranged to over 1.0 cm.

Example V Example IV was repeated except that commercial aluminum sheet(6.65 g.) was used in place of the powder and this was heated to1350-1450 C. in argon instead of nitrogen. After reaction temperaturewas reached, argon at 0.012 mo1e/hr., nitrogen at 0.025 mole/ hr. andtitanium tetrachloride vapor at 0.00044 mole/hr. were passed over thehot charge. The boat and the upstream surfaceof the aluminum residuewere covered with beautiful long golden fibers. Some individual fibersover 1 cm. long were observed. The golden fibers (0.025 g.) gave anX-ray difiraction pattern for titanium nitride face-centered cubiccrystals, 0 1.25 A. The X-ray pattern of the golden fibers was notchanged by treatment with 48% hydrofluoric acid. In addition to thegolden fibers, 0.15 g. of gray fibers was formed upstream (X-raypattern: face-centered cubic cell, 0,, 4.25 A.). The fiber diameter was0.10 to 3.0 microns.

Example VI Example V was repeated except that TiCL; vapors (0.0029mole/hr.) and argon (0.050 mole/hr.) were first passed over hot titaniumsponge (to give TiCl and TiCl before mixing'with nitrogen and passingover the aluminm charge. Golden fibers of TiN formed on upstreamsurfaces of boat. White and gray fibers having the X-ray pattern ofaluminum nitride formed on downstream sur faces of boat and itscontents.

Example VII Calcium nitride (Ca N- 7.43 g.) was heated in an Alundumboat in a 1%" LD. mullite tube in N to 12451425 C. Nitrogen (0.028 mole/hr.) and TiCl vapor (0.0013 mole/hr.) were passed over the heated chargefor 4 hours. A deposit of 0.19 g. of gray-brown fibers giving the X-raydiffraction pattern of TiN was removed from the upstream end of the boatand the adjacent walls of the mullite tube. This fiber diameter is0.044.0 microns with a length of up to 8 mm.

Example IX A mullite tube (1%." ID. x 30" long) was heated to 1330-1425C. at its mid-section. Titanium tetrachloride (0.0025 mole/hr.) andhydrogen (0.020 mole/hr.) were introduced to the hot zone through avitreous silica tube from the left end of the mullite tube and ammonia(0.015 mole/ hr.) was introduced through a similar tube from the right.Gaseous products were exhausted from the right end of the mullite tube.The flow of gases was maintained for 6.17 hours. A black and gray scale(0.11 g.) was recovered from the tube along with 10 mg. of long goldenfibers of TiN (over 1 cm. long). One of these fibers was found to behexagonal in cross section (11.6,u. across corners of the hexagon) andwas demonstrated to be capable of supporting a 50 g. weight Withoutbreaking. This is equivalent to a. tensile strength for the fiber inexcess of 1,000,000 lb./in. A fiber 10 microns in diameter when bent toa 0.5 mm. radius of curvature, showed complete and almost instanteousrecovery without fracture of permanent deformation even aftermaintaining it as a coil for some time. Such recovery effects have notbeen found in single crystal metal fibers of comparable dimension. Theelectrical resistivity was 20 microohms-cm.

Example X Example IX was repeated with the following conditions: Mole/hr. TiCl 0.0140 NH, 0.105 H 0.080

Temp, 13701470 C. Duration of run, 3.25 hours.

A doughnut-shaped deposit (1.01 g.) of crude golden TiN fibers formednear the hottest zone of the mullite tube.

Analysis: Found-N, 13.06; Ti, 50.53, 51.35; Si, 7.13%.

Example XI Titanium tetrachloride (0.0019 mole/hr.) and argon (0.035mole/hr.) were passed over hot titanium sponge packed in a quartz tubeand introduced through this tube to the hot zone of a surroundingmullite tube (1% LD.) at 1230-1310 C. Ammonia (0.015 mole/hr.) wasintroduced into the mullite tube around the quartz tube. Golden fibers(1.01 g.) formed near the tip of the quartz tube. The X-r-ay diffractionpattern of these fibers indicated TiN plus a trace of tit-quartz.Treatment of a 1.0 g. sample of these fibers with 48% HF and a smallamount of H 0 left a solid residue of 0.51 g. of golden fibers. TheX-ray difiraction pattern of these golden fibers gave a face-centeredcubic cell constant of a 4.235 A. The extracted fibers contained 16.79%nitrogen and 77.21% titanium. The fiber diameter was 0.5-6.0 microns.

Example XII An alumina boat containing 2.36 g. of TiO was heated in astreamof N in a 1% LD. mullite tube to a maximum temperature of 1420" C.TiCl (0.0023 mole/hr.) and N (0.045 mole/hr.) were passed over the hotTiO for 6.58 hours. At the end of the experiment the surface of the TiOparticles in the boat was covered with needles and fibers of TiN growingoutward from the surface.

This invention provides a new form of titanium nitride, namely, afibrous titanium nitride having high 1 tensile strength and a highdegree of flexibility. The

fibers have a diameter of up to about 10 microns and are usually between0.05 and 5 microns. The length of the individual fibers is at least 10times their diameter and generally -1000 times the diameter. Theflexibility exhibited by the fibers is outstanding. As shown in ExampleIX, a relatively thick fiber could be bent quite sharply and makecomplete recovery.

In addition to these outstanding properties, the titanium nitride fibersgenerally have an attractive color.

The fibrous form of titanium nitride is obtained by passing a mixture oftitanium tetrachloride with nitro gen in the presence of a reducingagent at a temperature of at least 1225 C. and preferably over 1300 C.in the presence of siliceous material. The mixture is readily obtainedby passing nitrogen through titanium tetrachloride prior to introductionin the hot furnace.

The titanium tetrachloride is a readily available volatile material.Nitrogen can be employed as gaseous nitrogen. In addition to nitrogenitself, ammonia can be used to. react with titanium tetrachloride togive titanium nitride. The reducing agent can be solid or gaseous, i.e.a reducing atmosphere. Suitable reducing agents are elements such ashydrogen, carbon, aluminum,

magnesium, calcium, titanium, iron, silicon, sodium, etc. Lower valentoxides, e.g., TiO, SiO, A1 0, etc., or lower valent halides such as SiClTiCl and TiOl can also be used. Other reducing agents include metalnitrides such as aluminum nitride and magnesium nitride, metal hydridessuch as calcium hydride, lithium aluminum hydride and titanium hydrideas well as carbides such as silicon carbide. It is preferred that thereducing agent gives by-products which are volatile under the reactionconditions. The particularly useful reducing agents are aluminum,magnesium, calcium for their nitrides), carbon and ammonia. In thesiliceous ceramic reactors employed, these reducing compounds arecapable of producing SiO under the reaction conditions. The lattercompound is an effective and important intermediate for the productionof titanium nitride fibers.

The fibrous titanium nitride is formed when the titanium tetrachlorideand nitrogen are passed slowly over a reducing agent or admixed in areducing atmosphere in the presence of a siliceous material heated to atemperature of at least 122S C. Rapid rates of gas flow inhibit fiberformation and the linear velocity of gaseous material should not exceed300 linear cm./hour when measured as an input at standard temperatureand pressure.

The temperature of the reaction vessel should be at least 1225 C. Themaximum temperature depends upon the highest that the equipment willmaintain without failure. Temperatures employed usually are not much inexcess of 1500 C. and preferably 13004500 C.

In general the container employed in heating the reactants is a ceramicand generally a siliceous refractory. Fiber formation takes place in thepresence of a siliconcontaining material. Small amounts of a fibrousform of an inorganic silicon compound are generally present with thefibrous titanium nitride. However, the fibrous product is composed oftitanium nitride in major amounts.

The fibrous product is removed by mechanical means from the refractory.Any fibrous inorganic silica compounds can be removed from the fibrousnitride by treat ment with hydrogen fluoride.

Identification of the fibrous titanium nitride is accomplished by X-raydiffraction pattern data.

The new titanium nitride fibers have extremely good thermal stability,inertness and strength. Mats or felts of the fibers are readily obtainedby suspending the fibers in a viscous liquid such as glycerine followedby filtration to remove the dispersing liquid. The mats are useful asfilters, e.g., in air to remove solids or to remove bacteria fromsolutions. They are good thermal insulators, especially where extremelyhigh temperatures are involved. The fibers can be incorporated inplastics to give increased stiffness and tear strength. They arereinforcing agents for fibers or papers. Finely ground fibers oftitanium nitride when incorporated with oils such as silicone oilproduce thick greases useful as lubricants.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An inorganic asbestos-like flexible feltable fiber consistingessentially of titanium nitride and characterized in that said fiber hasa diameter of less than about 10 microns and a flexibility sufficient topermit bending around a mandrel of 1 mm. diameter without breaking.

2. An inorganic asbestos-like flexible feltable fiber of high tensilestrength consisting essentially of titanium nitride and characterized inthat said fiber has a diameter of between 0.05 and 5 microns with alength of at least times its diameter and a flexibility sufiicient topermit bending around a mandrel of 1 mm. diameter without breaking.

3. As a new inorganic fibrous material, the asbestos like form oftitanium nitride characterized in that it is in the form of flexiblefeltable fibers each having a diameter of less than about 10 micronswith a length of at least 10 times its diameter and a flexibilitysuflicient to permit bending around a mandrel of 1 mm. without breaking.

4. As a new inorganic fibrous material, the asbestoslike form oftitanium nitride characterized in that it is in the form of flexiblefeltable fibers of high tensile strength each having a diameter ofbetween 0.05 and 5 microns with a length of at least 100 times itsdiameter and a flexibility sufiicient to permit bending around a mandrelof 1 mm. diameter without breaking.

5. Process of preparing asbestos-like flexible feltable fibersconsisting predominantly of titanium nitride which comprises passing andreacting titanium tetrachloride and nitrogen in a slowly moving streamhaving a linear velocity of not more than 300 linear cm./hour, measuredunder standard conditions of tempera-ture and pressure, in contact witha siliceous material heatedto a temperature of at least 1225 C. andinthe presence of a reducing agent, and removing the asbesos-like flexiblefeltable fibers thus formed consisting predominantly of titaniumnitride.

'6. Process of preparing asbestos-like flexible feltable fibersconsisting predominantly of titanium nitride which comprises passing andreacting titanium tetrachloride and nitrogen in a slowly moving streamhaving a linear velocity of not more than 300 linear cm./hour, measuredunder standard conditions of temperature and pressure, in contact withaluminum silicate heated to a temperature of 1300 to 1500 C. and in thepresence of carbon, and removing the abestos-like flexible feltablefibers thus formed consisting predominantly of titanium nitride.

7. A composition of matter-comprising a mat of titanium nitride flexiblefeltable fibers each having a diameter of less than about 10 micronswith a length of at least 10 times its diameter and a flexibilitysutficient to permit bending around a mandrel of 1 mm. diameterwitthoutbreaking.

References Cited in the file of this patent UNITED STATES PATENTS OlsonJan. 7, 1947 Alexander Feb. 8, 1949 OTHER REFERENCES Mellor:Comprehensive Treatise on Inorganic and

1. AN INORGANIC ASBESTOS-LIKE FLEXIBLE FELTABLE FIBER CONSISTINGESSENTIALLY OF TITANIUM NITRIDE AND CHARACTERIZED IN THAT SAID FIBER HASA DIAMETER OF LESS THAN ABOUT 10 MICRONS AND A FLEXIBILITY SUFFICIENT TOPERMIT BENDING AROUND A MANDREL OF 1 MM. DIAMETER WITHOUT BREAKING.